Question
Find the maximum value of the function f(x) = x³ –
3x² – 9x + 20.Solution
We are given the function: f(x) = x³ – 3x² – 9x + 20 We need to find its maximum value. Differentiate f(x): f′(x) = 3x² – 6x – 9 Set f′(x) = 0: 3x² – 6x – 9 = 0 Divide all terms by 3: x² – 2x – 3 = 0 Factor: (x – 3)(x + 1) = 0 ⇒ x = 3, –1 Differentiate again: f″(x) = 6x – 6 Now test the sign of f″(x) at critical points:
- At x = –1:
f″(–1) = 6(–1) – 6 = –12 → Negative ⇒ local maximum - At x = 3:
f″(3) = 6(3) – 6 = 12 → Positive ⇒ local minimum
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